1. Field of the Invention
This invention relates to a sheet conveying apparatus, and more particularly to a sheet conveying apparatus that stacks sheets being conveyed on a stacking section and then discharges a bundle of the sheets stacked on the stacking section.
2. Description of the Related Art
Conventional image forming apparatuses such as a copying machine include a type that is capable of operating in a cover mode, an interleaved sheet mode, and the like, in which insert sheets, such as covers, are added to sheets having images formed thereon. In these modes, the image forming apparatus is controlled such that a sheet supplied from a cassette or a feed tray provided in the image forming apparatus can be inserted into a sheet bundle as the first page, the final page, or an intermediate page. The term “insert a sheet” is intended to also mean “add a sheet to” in the case of the sheet being a cover or a back cover, throughout the specification. Therefore, it is possible to carry out processing for forming a sheet bundle out of sheets fed from a single feed cassette and another type of sheets inserted into the sheets from the single feed cassette. More specifically, it is possible to feed inserting sheets (hereinafter referred to as “insert sheets”) such as a “cover”, an “interleaved sheet” and a “back cover”, from other feed cassettes and insert the insert sheets into sheets having images formed thereon, to thereby form a sheet bundle. Further, similar inserting processing can be performed by feeding insert sheets from a special tray having insert sheets stacked thereon.
In this case, the processing relating to insert sheets is a mere sheet conveying operation, and therefore it is possible to freely set both inserting positions of insert sheets in a sheet bundle, i.e. inserting places where insert sheets are to be inserted, and the number of insert sheets to be inserted at each inserting position, as desired. Further, the sheet bundle having the insert sheets inserted therein can be subjected to sheet bundle processing by a finisher or the like which is incorporated in the image forming apparatus, i.e. post-processing including bundle discharge processing for discharging the sheet bundle, staple processing for stapling the sheet bundle, folding processing for folding the sheet bundle, and bookbinding processing for bookbinding the sheet bundle. Hereinafter, operation modes for inserting an insert sheet as a “cover”, an “interleaved sheet” or a “back cover”, from an insert sheet cassette will be generically referred to as “the interleaved sheet mode”.
To supply insert sheets from a cassette, in timing in which an insert sheet is to be inserted, the insert sheet is fed from the cassette to the same conveying path along which a sheet on which an image is to be formed is conveyed, and then the supplied insert sheet is discharged via the conveying path. In an intermediate portion of the conveying path, there is arranged a fixing section, and the insert sheet passes this section as a sheet on which an image is to be formed does.
When a color image-printed original is used as an insert sheet, the insert sheet receives thermal pressure as it passes the fixing section, which can degrade the quality of the printed image. Further, with recent diffusion of personal computers, more and more color images have come to be used, and color copy sheets/color print sheets have come to be used as insert sheets more frequently. However, color copy sheets supplied from a cassette can have oil or the like adhering to surfaces thereof, to deteriorate the sheet conveying performance of the sheet feed mechanism, which can considerably degrade reliability of sheet conveying operation of the apparatus.
Another type of image forming apparatus has emerged in which an insert sheet feeder for supplying insert sheets is provided in a finisher so as to supply insert sheets from the finisher. Apparatuses of this type have been proposed e.g. in Japanese Laid-Open Patent Publications (Kokai) No. 60-180894, No. 60-191932, and No. 60-204564. More specifically, according to the apparatuses disclosed in these patent publications, insert sheets are each supplied from the insert sheet feeder to the finisher in desired timing, and then conveyed to an intermediate tray within the finisher to be received and stacked on the intermediate tray. Sheets discharged from the main unit of the image forming apparatus are also introduced into the finisher to be received and stacked on the intermediate tray. To enable the apparatus to perform such operation, it is necessary to stack in advance insert sheets in a sheet container of the insert sheet feeder in the order corresponding to the order of pages dependent on the contents images to be formed and in a number of sets corresponding to the number of copies to be produced.
However, the above prior art suffers from the following problems. In the conventional image forming apparatus, to insert sheets using the insert sheet feeder in the interleaved sheet mode, it is necessary to reliably feed the insert sheets one by one from the feeder into the finisher. However, the insert sheets include a wide variety of types and usually have a variety of images formed thereon, thus differing in stability from transfer sheets used for having images formed thereon, in performing automatic operation for separating and conveying each sheet. For example, insert sheets can cause so-called “multiple feed” in which a plurality of insert sheets which should be fed one by one are fed simultaneously. This “multiple feed” makes the disorder of the sequence of pages of sheets of the present and following bundles.
Another conventional image forming apparatus has been proposed in which a desired number of copies is set through the operation of an operating section, and the formation of images is continuously carried out until the output of the copies is completed. In this apparatus, when multiple feed of insert sheets occurs, sheet bundles formed after the occurrence of the multiple feed all have insert sheets inserted in wrong places, which causes waste of sheets, time, power consumption, and so forth that are required for outputting the sheet bundles.
Further, still another type of image forming apparatus has been proposed which temporarily stops outputting whenever each sheet bundle is completely output, to thereby enable the user to check whether or not proper output has been performed. In this case, it is possible to detect multiple feed earlier than when the formation of images is continued without stopping any output after multiple feed has occurred. However, e.g. in the case of an output bundle of a large number of sheets, even if the user visually detects multiple feed, since the operation is continued until the output is temporarily stopped, wasteful feed of insert sheets inevitably occurs.